1. Field of the Invention
This invention relates to an improved system and method of using corrected signals from a Global Positioning System to perform precision surveying.
2. Discussion of the Prior Art
U.S. Pat. Nos. 5,519,620 and 5,602,741 describe some prior art Global Positioning systems. The Global Positioning System (GPS) is a satellite-based system that provides users with real-time position information originally intended for navigational purposes. This system is comprised of a constellation of 24 satellites with sufficient redundancy such that about 6 satellites are typically visible to users at any time or location on the planet. Transmissions from at least 4 satellites must be received in order for users to fix the latitude, longitude and elevation of their position. These satellites were launched by the United States Government Department of Defense. For national security reasons, the government purposely prevents commercial users of the system from attaining the same level of positional accuracy that government users can achieve. This is called Selective Availability (SA). With SA in place, commercial users can fix their position within a radius of 100 meters.
Differential GPS (DGPS) is a technique, enabled with the addition of a wireless data link, which improves commercial GPS positioning capability by several orders of magnitude. This capability makes DGPS technology practical for many other applications, particularly precision survey, which typically demands sub-meter accuracy. To employ DGPS in a local area, the user must position a GPS satellite receiver at a known location and leave it stationary. This set up is usually called the base (or reference) station. An example of a base station is illustrated in FIG. 1. Satellite signals are received with GPS antenna 10 and the signal is sent to the GPS base station receiver 15 through RF cable 11. Typical GPS receivers are tasked with using the satellite signals to determine their position. GPS base stations already know their position and are therefore capable of performing the traditional positioning task “backwards” in order to compute and assign a correction to individual satellite transmissions that are within the base station's field of view. These corrections are sent to the radio modem transceiver 13 through data cable 19. The radio modem transceiver 13 is connected to the radio antenna 12 through RF cable 20. Power is supplied to the base station GPS receiver and radio modem transceiver from DC battery 17 via power cables 18.
In addition to the deliberately induced SA error, satellite transmissions are also adversely affected (essentially delayed in time) by the atmosphere, ionosphere and by reflected signals that are time delayed as a result of not traveling to the GPS receiver on a straight path. All of these error sources are dynamic which means that the corrections computed by the base station are constantly changing and must be transmitted to the rover units at a continuous rate of typically once per second.
To perform precision positioning, e.g. survey activity, with DGPS, it is necessary to also have at least one rover (or mobile) GPS receiver marking the points of interest in the same local area in which the base is located. An example of a typical rover unit is illustrated in FIG. 2. These rover units simultaneously receive the same satellite signals that the base is receiving, through their GPS antennae 210. This signal is conducted to the GPS rover unit satellite receiver 216 through RF cable 211. The position capability of the rover unit is enhanced with the value of the correction to be attributed to each satellite signal that it receives. Radio modem antenna 212 receives the RF signal representing this information that was transmitted by base station radio modem transceiver 13. This signal is conducted to the radio modem receiver 214 via RF cable 220. This data is then sent through data cable 219 to GPS rover unit satellite receiver 216. The system combines the received satellite signals with the satellite corrections that are broadcast to it on the wireless link and is thereby able to compute its position to within a few centimeters. The surveyor operating the rover unit then marks this position and moves to the next.
Survey activities are conducted outdoors and sometimes under extreme environmental conditions. The equipment must be rugged, simple to operate and very reliable. Rover unit equipment must be lightweight, portable and power efficient due to the weight of batteries and the inconvenience of recharging them. The generally accepted practice involves placing the rover unit radio modem receiver, GPS rover unit satellite receiver, and DC battery in a backpack with the radio modem antenna and GPS antenna left external. The radio antenna works best when it is elevated (this increases radio range) and the GPS antenna works best when it has a maximized view of the horizon (this increases the number of satellites that can be received). Hand-held, or backpack-mounted, poles are employed to elevate these antennae as required. It is always desirable to reduce the number of cables (e.g. 211, 220, 218, 219) in the system since these are inconvenient to attach and detach and are invariably sources of reduced performance and system failure.
Base station equipment is typically stationary and mounted on a tripod on high ground for best radio range and satellite field of view. The radio modem typically has a high power transmitter since its main function is to broadcast satellite corrections to the rovers. Even though this station is set-up once at the beginning of the job and left unattended until the end of the job, users prefer this equipment to be simple and highly integrated. When a base station is moved from one location to another it will typically be tossed roughly into the back of a pick up truck which is then driven across an unpaved surface to the next site.
Using narrow band technology for the radio modem, the range of these data links can vary from 2 to 50 miles depending on the terrain and transmission power being used. Narrow band radios have longer wavelengths than most of their spread spectrum counterparts. This gives narrow band a longer line of sight range and better penetration through foliage than is realized by spread spectrum radios in the field. Narrow band radios have the disadvantage of requiring that a license be obtained from the Federal Communications Commission (FCC), or local regulatory body, prior to operation.
Surveyors using DGPS are members of the spectrum community as they share the radio waves with other licensed users, each with different communication requirements. For reasons of precedence and public safety, the FCC gives voice users priority over data (e.g. DGPS) users. To ensure this practice is followed the FCC requires data transmissions to employ Carrier Sense Multiple Access (CSMA) which is an algorithm that first senses whether another transmitter is presently transmitting before it begins its own, potentially interfering, transmission. The FCC also requires transmitters of periodic data to identify themselves every 15 minutes with a Morse code transmission of their FCC call sign. This practice facilitates the reporting of irresponsible transmitters back to the FCC. The FCC has the authority to issue substantial fines and even confiscate the equipment of illegal transmitters.
Manufacturers of GPS receiver equipment have recognized the importance of integrating the radio modem inside their GPS receiver enclosures. Thus far this integration has been limited to the rover systems only and has involved radio modems that are capable only of receiving (e.g. they have no transmitters) data transmissions. One particular example has been reduced to practice by Trimble Navigation Limited (TNL) and is illustrated in FIG. 3. It has the GPS receiver 314, a GPS antenna 310, a receive-only radio modem 316, and an internal radio modem (slot antenna type) antenna 312, all integrated into a single enclosure 321 that mounts on top of a hand-held pole with threaded insert 328. The internal antenna 312 is constructed of a lamination of polymide insulating material and electrically conductive copper tape material. FIG. 4 illustrates the internal antenna 312 in an unwrapped view. The construction of this antenna is such that the receiving elements of the antenna are the areas where there is no copper present 423 surrounded by conductive area 424 where the copper is present. A planar coaxial lead 425 allows the signal to be directed from the antenna to the RF cable 320 shown in FIG. 3. When wrapped, the antenna ends are electrically connected by conductive areas 428, 430. FIG. 5 illustrates the direction gain properties of this antenna that is seen to have 3 dB of variation in its implementation. DC Power is supplied to the system through the pole (not shown) which contains two insulated conductors that make electrical contact between the contacts 322 in the enclosure 321 and the batteries that are placed in the bottom of the pole. These batteries power both the rover GPS satellite receiver and the rover radio modem receiver and provide approximately 4 hours of usable operation in this implementation.
The TNL embodiment described above provides a level of integration that eliminates the need for the surveyor to connect external data cable 219 between the modem and GPS receiver. It additionally obviates the external RF cables 211, 220 between the two antennae and their respective receivers 216, 214. Surveyors have complained that this particular system is top-heavy and unwieldy, being placed at the top of a hand-held pole. This is at odds with the requirement that to achieve precise survey results the GPS antenna must be held stationary since all satellite measurements will be referenced to the center of the surface of the GPS antenna. If the antenna is swaying during these measurements their precision will be reduced. Surveyors prefer the use of a backpack for carrying the heavier components of the rover system.
At present, base station systems are comprised of GPS and radio modem transmitters packaged in separate mechanical enclosures and electrically connected (only) with an external serial data cable. Once set up, the base stations are programmed to transmit their corrections every one second or so continuously without any user intervention. When the radio channel is being shared among many users the DGPS corrections will be buffered up in the memory of the base station radio modem while it continues to sense for the frequency to be clear so that it may transmit the buffered data to one or more rover units. When the channel is finally sensed as clear by the base station transceiver it will transmit a long stream of data comprised of all of the corrections that were collected while the channel was occupied by another user. However, due to the temporal nature of these corrections, the only data that is of any value to the rover GPS receiver will be that set which was most recently calculated by the base GPS receiver.